Phase-shift masks are used in a variety of photolithographic applications to form semiconductor integrated circuits and light-emitting diodes (LEDs). Phase-shift masks differ from conventional chrome-on-glass masks in that the transparent regions in a phase-shift mask have a relative phase difference while in a chrome-on-glass mask, the transparent regions all have the same relative phase. The benefit of having select phase differences for the transparent regions in a phase-shift mask is that the transparent regions can be configured so that the electric field amplitudes add in a manner that result in a sharper image intensity (image contrast). This in turn results in an increased imaging resolution when printing features in photoresist.
An example phase-shift mask can include alternating phase-shift regions, i.e., a periodic pattern of 0 degree and 180 degree (π) phase regions. Such a phase-shift mask is useful for enhancing feature resolution of repeating patterns. However, the improved image contrast ceases at the edge (perimeter) of the lithography exposure field because the repeating pattern must terminate. As a result, the feature patterns formed adjacent to the field edge tend to be distorted by discontinuity in the otherwise beneficial phase interference. This issue has been addressed in the past by configuring the phase-shift mask and the imaging field such that the distorted features would print in a region on the wafer that ultimately would not be used to form the actual device. However, not all manufacturing applications have this flexibility, and therefore the conventional alternating-phase phase-shift mask cannot be employed.
One example manufacturing application where distorted features at the field edge can be problematic is in LED manufacturing. LEDs are becoming increasingly more efficient due to continuous improvements in LED fabrication and LED design. However, a general limitation on LED light emission efficiency is due to a total internal reflection of the light generated within the LED. For example, in a gallium-nitride-(GaN)-based LED, n-doped and p-doped GaN layers are supported by a semiconductor substrate (e.g., sapphire) having a surface. The n-doped and p-doped GaN layers sandwich an active layer, and one of the GaN layers has a surface that interfaces with air. Light is generated in the active layer and is emitted equally in all directions. However, GaN has a relatively high refractive index of about 3. As a result, there exists at the GaN-air interface a maximum-incident-angle cone (“exit cone”) within which the light exits the p-GaN-air interface, but outside of which light is reflected back into the GaN structure due to Snell's Law.
To improve LED light emission efficiency, certain LEDs have been fabricated with a roughened substrate surface. The roughened substrate surface scatters the internally reflected light, causing some of the light to fall within the exit cone and exit the LED, thereby improving the light emission efficiency of the LED.
In a manufacturing environment, it is desirable to have a controllable and consistent method of forming the roughened substrate surface so that the LEDs have an identical structure and identical performance. To this end, it is desirable that the roughened substrate surface be formed without the above-described feature pattern distortions.